“They entered one of the buildings and are engaged in heavy fighting with soldiers inside the complex.”

He said the assault began with a car bombing carried out by a member of Al-Qaeda’s Syrian affiliate Al-Nusra Front.

It is the first time that the rebels have managed to penetrate the complex.

Among the 250 people holed up inside are around 150 regime troops, including “high-ranking officers,” as well as their family members and some civil servants, according to Abdel Rahman.

Rebels seized Jisr al-Shughur on April 25, extending their gains in Idlib province, where they have also captured the provincial capital and a military base in recent weeks.

Government troops and militia have been battling to reach the hospital to relieve the siege.

They began their counteroffensive on Wednesday, as President Bashar al-Assad pledged those in the hospital would be rescued soon, and are now around two kilometres (little more than a mile) away, according to the Observatory.

They have been backed by air strikes against the rebels laying siege to the hospital, which state media said had killed “dozens of terrorists.”

It remains unclear how much food and ammunition those holed up inside the hospital complex have left.

More than 220,000 people have been killed in Syria since the conflict began with anti-government protests in March 2011 that spiralled into civil war after a crackdown by security forces.